Pressure-sensitive recording sheets which utilize the coloring reaction between electron-donating color formers and electron-accepting color developers are widely known as no carbon required papers or pressure-sensitive copying papers and examples thereof are described in U.S. Pat. Nos. 2,711,375, 2,712,507, 2,730,456, 2,730,457, and 3,617,334; and Japanese Patent Publication Nos. 18317/63, 1178/72 and 20972/72.
A pressure-sensitive copying paper is generally composed of an upper sheet (CB (coated back) sheet) having coated with microcapsules containing an appropriate electron-donating color former in a suitable solvent and a lower sheet (CF (coated front) sheet) having coated with an electron-accepting color developer, or is composed of the upper sheet, the lower sheet, and an intermediate sheet (CFB (coated front and back) sheet) having coated with the microcapsules and coated with the color developer on the other surface thereof. In one embodiment, the upper sheet is superposed on the lower sheet so that the coated faces thereof face each other, or in another embodiment, one or more intermediate sheet are inserted between the upper sheet and the lower sheet. When pressure is applied to the above-constructed pressure-sensitive copying paper by a typewriter or handwriting, the microcapsules are ruptured to release the color former with the solvent, which is then transferred on to the surface of the color developer-containing layer to form colored image.
Such pressure-sensitive copying papers find much utility in such business forms as vouchers and paper for computer printout.
The most important requirements that should be met by the pressure-sensitive recording sheet are that it has an excellent coloring property upon application of the impact upon typing or the pressure of writing and that it undergoes a minimum degree of processing smudge, which is the smudge resulting from handling during manufacture or subsequent conversion treatment.
Smudge preventing particles have been extensively used with a view to minimizing the occurrence of smudge which results from the application of pressure during conversion, and known smudge preventing particles include fine cellulose powders (U.S. Pat. No. 2,711,375), starch particles (British Pat. No. 1,232,347; and Japanese Patent Publication Nos. 1178/72 and 33204/73), glass beads (U.S. Pat. No. 2,655,453), and heat expandable polymer particles (Japanese Patent Application (OPI) No. 322013/73, with the term "OPI" as used herein referring to an "unexamined published Japanese patent application"). The use of these protective agents is effective in minimizing the processing smudge but, at the same time, they reduce the density of the reproduced color image. Commercially, this problem can be solved by applying increased amounts of microcapsules or increasing the content of the color former within the microcapsules, but from the viewpoint of cost-effectiveness, neither method is economically desirable.
Methods have been proposed for solving the problem by applying microcapsules onto the surface precoated with microcapsule protecting agents such as starch or water-soluble polymers (as in Japanese Patent Application (OPI) Nos. 111288/80 and 87908/73; and U.S. Pat. Nos. 3,565,666, 3,914,470 and 3,219,220), but in fact the improvement attained is not as great as desired.